Switch-mode power converters are widely used in various power conversion applications, such as single-phase and three-phase power factor corrected AC/DC rectifiers and DC/AC inverters. As the power levels of the converters are increased, multiple fast switching parallel-coupled semiconductor power switching devices are employed to accomplish system application requirements. In some cases, the power switching devices form main boost switches for a boost converter, which are simultaneously turned on and off.
Insulated-gate bipolar transistors (IGBTs) have much lower conduction losses as compared with field-effect transistors (FETs) and much faster switching capabilities and easier gate drive control when compared to bipolar junction transistors (BJTs) and gate turnoff thyristors (GTOs). The IGBT, with its faster switching speed and lower conduction losses, has become a preferred semiconductor switching device for use in high frequency and high power applications.
Unfortunately, the minority carrier switching devices, such as the IGBTs, do not share current well, especially when two or more IGBTs are parallel-coupled. The current-sharing capability of the switching devices is important for high frequency and high power applications where multiple power switching devices are coupled in parallel to handle the required current. The parallel-coupled IGBT switching devices do not share current well primarily because of the conduction characteristics of the mismatched devices and the negative temperature coefficient of the on-state resistances. The IGBT with the better conduction characteristics, i.e., lower voltage drop, carries a larger share of the load current to equalize its voltage drop with the other IGBT. This is the case even when the switching devices are mounted on a common heat sink. Experience and tests have indicated that the current unbalance between parallel-coupled IGBTs may be as high as 50 to 70%, at low currents. The poor current sharing of the switching devices significantly reduces the device silicon utilization due to the current imbalance and the device carrying more current will have a higher body temperature.
To overcome the poor current-sharing capabilities of the parallel-coupled IGBTs, more semiconductor devices are normally used to increase the device current margin. Unfortunately, increasing the number of devices also increases the likelihood of mismatching each device characteristics thereby increasing the complexity of printed wiring board (PWB) circuit layout and the overall cost. As a result of the unpredictable current-sharing capabilities of the parallel-coupled IGBTs, the overall system cost is increased and the reliability substantially reduced.
Accordingly, what is needed in the art is an improved method that provides balanced current distribution for parallel-coupled semiconductor power switches.